In recent years, various devices for after-treating diesel particulates have been developed for installation in the exhaust systems of diesel engines so as to curb dispersion of fine particles contained in diesel engine exhaust gas into the atmosphere. The exhaust gas treatment devices of this type in wide use are generally configured to use a filter for trapping particulates contained in the exhaust gas of a diesel engine. The filter for trapping particulates is configured to trap particulates by using a ceramic or metal filter element for separating the exhaust gas and particulates. Particulates trapped inside the filter therefore gradually accumulate until the filter ultimately becomes clogged and passage of exhaust gas becomes impossible.
The practice has therefore been to use a filter control system that estimates the quantity of particulates accumulated in the filter based on the pressure differential across the filter or by integrating the quantity of particulates generated by the engine per unit time based on an engine operating condition, and that performs engine operation control for increasing the exhaust gas temperature when the estimated quantity of particulates reaches a prescribed level, thereby burning the particulates and regenerating the filter.
In this type of filter, a phenomenon of accelerated oxidation arises at the zone of the particulates accumulated in the filter that is most exposed to high-temperature gas. It is said that this results in part of the layer of accumulated particulates being maintained in a state in which the exhaust gas can always pass easily, so that the rise in exhaust resistance in the filter stops and the level of the exhaust resistance stabilizes at a certain value, notwithstanding that accumulation of particulates continues at other parts of the filter. When such a phenomenon arises, increase in particulates cannot be detected by the conventional method based predominantly on differential detection, even when a large quantity of particulates accumulates in the filter. Since the timing of forced filter regeneration is therefore delayed, the regeneration starts at a quantity of particulates exceeding the allowable range of the filter, so that the oxidation reaction of the particulates progresses intensely and the filter is liable to suffer fusion damage.
On the other hand, in the case where an arrangement is adopted in which the quantity of particulates that the engine generates per unit time is calculated using map data measured beforehand in a standard engine and the quantity of accumulated particulates in the filter is estimated by integrating this value, a discrepancy arises between the quantity of particulates generated per unit time measured beforehand in the standard engine and the actual quantity of particulates generated, owing to, among others, difference between individual engines, properties of the fuel used, and degradation of the fuel injection nozzles and other components. In addition, since the accumulation quantity map used to estimate the quantity of accumulated particulates is based on the quantity of accumulated particulates measured with the engine in a normal operating state, the accumulated quantity cannot be accurately mapped during a transient operation state. As a result, there is a problem of it being difficult to estimate the quantity of accumulated particulates with high accuracy.
Thus, it has been hard to estimate the quantity of particulates accumulated in a filter with high accuracy and difficult to conduct filter regeneration at appropriate timing, and this has led to such inconveniences as fusion damage of the filter during regeneration and decrease in fuel efficiency.
An object of the present invention is to provide a filter control method and system that overcomes the foregoing problems of the prior art.